Well it could have been any one of the problems posted here. Thanks to all for suggestions.

It turns out the dealer was right after all. The 'new' monitor I fitted didn't work initially, but the dealer got it going again. I wasn't clear on why it didn't work, but the service person muttered something obscure about needing to recode the unit to 'accept' it's new home. The one I fitted came from an X5 so perhaps it needed a nudge to get it going.

As a side-effect, my TrafficMaster box no longer works either, but with a Mk4 GPS I don't really care about that. I'm just glad it wasn't causing the screen to go blank because its hard-wired and you can't quick-bypass it out like you can with a faulty TV monitor.